Abstract:

The resident ecotype of killer whales (Orcinus orca) in the waters of British Columbia and Washington State has a strong preference for Chinook salmon even in months when Chinook are 5%–10% of the salmon population. The foraging behavior of ki ller whales suggests that they rely on echolocation to detect and recognize their prey. In order to determine possible cues in echoes from salmon, a series of backscatter measurements was made at the Applied Physics Laboratory, University of Washington Facility on Lake Union, on three different salmon species using simulated killer whale echolocation signals. The fish were tied to a monofilament net and rotated while echoes were collected, digitized, and stored on a laptop computer. Three transducer depths were used; same depth, 22o, and 45o above the horizontal plane of the fish. Echoes were collected from five Chinook, three Coho, and one Sockeye salmon. Radiograph images of all the specimens were also obtained to examine the swim bladder shape and orientation. Results show that the echo structure from similar sized but different species of salmon were different and probably recognizable by foraging killer whales. The results also suggest that a broadband echo-sounder pointing downward could be used to discriminate salmon species.